【leetcode刷题笔记】Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

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题解：Using two pointers head and tail, head keeps moving forward till it points to an alpha or number; tail moves backward till it points to an alpha or number, then judge if what head points to equals what tail points, if it doesn't equal, return false; Else cotinue moving head and tail to compare characters bewteen them.

We also need a function isValid(char c) to judge if char c is a number or alpha.

The code is below:

 public class Solution {
     private boolean isValid(char c){
         if(c >= 'a' && c <= 'z')
             return true;
         if(c >= '0' && c <='9')
             return true;

         return false;
     }

     public boolean isPalindrome(String s) {
         s = s.toLowerCase();

         if(s == null || s.length() <= 1)
             return true;

         int head = 0;
         int tail = s.length() - 1;

         while(head < tail){
             while(head < tail && !isValid(s.charAt(head)))
                 head++;
             while(tail > head && !isValid(s.charAt(tail)))
                 tail--;
             if(s.charAt(head) != s.charAt(tail))
                 return false;
             else {
                 head++;
                 tail--;
             }

         }

         return true;
     }
 }